The present invention relates to fuel assemblies for nuclear reactors, and in particular to structure carried by the assemblies for maintaining proper inter-assembly spacing and alignment.
The nuclear reactor core of a pressurized water power reactor typically consists of a multiplicity of elongated, square fuel assemblies, each containing a square array of some two hundred individual fuel rods. The rods are supported in the assembly by a plurality of longitudinally spaced, transversely oriented grids which in turn are affixed to rigid, longitudinally extending tubes or columns. These support tubes are secured in end fittings which are seated directly or indirectly to support plates connected to the reactor vessel. A major objective of this fuel arrangement is to maintain proper spacing between individual fuel rods within each assembly, and between the assemblies.
Various normal conditions occur during core operation which alter the as-built, nominal spacings. One obvious condition is coolant temperature, which typically varies from a spatially uniform value of about 100.degree. F. (38.degree. C.) during refueling, to a spatially varying value in the range, for example, of 560.degree. F.-620.degree. F. (293.degree. C. to 326.degree. C.). In reactors using a steel vessel and support plates, and substantially all-Zircaloy fuel assemblies, a difference in coefficient of thermal expansion introduces a spatial dependence on the fuel assembly spacing. Another normally present effect is the dimensional change in the assemblies with time, as a result of irradiation-induced creep and growth. These temperature and irradiation effects have been accommodated to a tolerable degree with careful design and manufacturing.
Other conditions, however, have been found to require additional attention and have posed problems less amenable to easy solution, particularly in cores having all-Zircaloy assemblies. For example, the core must be capable of withstanding the load imposed by hypothesized seismic disturbances which, being primarily in the lateral direction, could cause assemblies to impact and damage one another. Therefore, to reduce impact loading, the prior art has, to a large extent, found it desirable to minimize the interassembly spacing during reactor power operation, yet provide adequate clearance to permit removal and rearrangement of assemblies during refueling. A commonly proposed technique uses bimetallic structures carried by or acting against the fuel assemblies to provide a "tighter" core during power production than during refueling. One such example is shown in U.S. No. 4,059,483 "Nuclear Fuel Assembly Seismic Amplitude Limiter," where the midplane of every assembly carries a stainless steel "seismic grid" rather than a conventional all-Zircaloy spacer grid.
Recently, another operating condition known as fuel assembly bowing has been observed. Bowing produces a pronounced, permanent curvature in the intermediate portions of the fuel assemblies, which worsens as burnup increases during a fuel cycle. When accumulated over several assemblies across the core midplane, bowing may impair the neutronics-related safety margins during power operation, and impede fuel assembly removal or replacement during refueling outages.
Although some of the prior art fuel assembly designs intended for use in areas of high seismic activity might, to some extent, incidentally reduce the adverse effects of bowing, they are deficient in several respects, especially when used in all-Zircaloy fuel assemblies:
1. Most rely on bi-metallic structure, which is expensive to fabricate and usually introduces unwanted, strongly neutron absorbingmaterial in the core.
2. The applied inter-assembly forces are either too weak, or effective over insufficient area, to adequately counteract the bowing forces.
3. The burnup dependence of the bowing phenomenon is not adequately accounted for.
If the spacing between assemblies is simply reduced to a value where bowing is limited to an acceptable value not only is there difficulty in initial fuel assembly loading but because of irradiation growth of grids as a function of time (burnup) it is likely that the core could be "locked up" so that fuel removal would be extremely difficult. Therefore what is required is a grid which provides the desired close spacing between fuel assemblies, but which is resilient enough to account for irradiation induced grid growth and normal refueling loads.